[stella] AtariVox/SaveKey question

[bob.montgomery at thomson.com]

Hi Thomas,

So the acknowledge bit is only necessary when reading, and I just make
sure the overflow-flag is clear when I call i2c_rxbyte.

Correct?

Thanks,

-Bob

-----Original Message-----
From: stella-bounces at atari2600.org [mailto:stella-bounces at atari2600.org]
On Behalf Of Thomas Jentzsch
Sent: Tuesday, June 05, 2007 11:49 AM
To: Atari 2600 programming list
Subject: Re: [stella] AtariVox/SaveKey question

Bob Montgomery wrote:
> I can't quite figure out how the overflow-flag should be set -
> can you help me out?  When do I need an acknowledge bit?  Is it
> ever necessary?

Here is what Alex wrote me (I had the very same questions :-):

"Normally, after reading a byte, we transmit a 0 bit for acknowledge.
This
tells the EEPROM to continue transmitting data on future pulses of SCL. 
However, once we have read all the bytes we want, a 1 is transmitted for

"not-acknowledge" instead, which stops the chip from sending any more
data. 
(It's not possible to issue a "stop" signal while the EEPROM is trying
to 
transmit.  The I2C protocol is a little clumsy.)

When writing the driver code, I found it simpler to put the ACK part at
the 
start of i2c_rxbyte (to acknowledge a *previous* byte read).  So we only

transmit the 0 when we *know* that more data is going to be read.  (The
V 
flag indicates whether a byte has been read previously - so we don't 
transmit ACK before reading the first byte.)  The Not-ACK is then done
in 
i2c_stopread.

If you destroy the V flag during EEPROM reads (which I imagine will be
the 
case if you're spreading reads across frames), and if you always read
the 
same number of bytes, it might be simpler to have 2 different "rxbyte" 
routines.  A normal "rxbyte" which always transmits an ACK (0) at the
end, 
and a "rxlastbyte" which transmits a NACK (1) which is only used when 
reading the last byte of data."

Hope that helps.

Have fun!
Thomas                            
_______________________________________________________
Thomas Jentzsch         | *** Every bit is sacred ! ***
tjentzsch at web dot de |


_______________________________________________
Stella mailing list
Stella at atari2600.org
http://atari2600.org/mailman/listinfo/stella